The present invention relates to a transmitting apparatus and a transmitting system for transmitting a signal composed of main data Dt and a TMCC (Transmission and Multiplexing Configuration Control) signal having a SYNC pattern indicating a header portion and a bit pattern indicating the setting information of the main data Dt. The present invention also relates to a receiving apparatus for receiving the signal and a signal structure of the signal.
In recent years, the OFDM scheme has begun to be used as a multiplexing scheme for digital transmission. The OFDM scheme is a transmission using a multiplicity of carriers and uses a carrier called TMCC carrier for sending ancillary data in addition to a carrier for sending the main data.
The main data is a transport stream (hereinafter referred to as TS) in which a video signal and an audio signal are compressed by an MPEG process. Until several years ago, an image and a voice had been transmitted by a method using an analog FM. In the analog FM, the S/N of the image and the voice changes with the level of the electric field strength received. In the mobile transmission such as the live broadcast of a marathon accompanied by a sharp change of the electric field strength level, the image telecast tends to have a low quality with considerable noises and disturbances. In the digital transmission such as OFDM, on the other hand, the information is digitized while at the same time error correction processing is used. Therefore, even in the case where the level of the received electric field strength changes, an image of the same quality can be transmitted on the spot in the range where the error correction is effective.
Once the electric field strength is reduced below a critical value, the error cannot be corrected any longer, and the image transmission becomes impossible.
This critical value is inversely proportional to the amount of the data transmitted.
For example, in a 64-QAM (Quadrature Amplitude Modulation) having the coding rate of 5/6 of the convolution code with a large transmission rate of 60 Mbps, the critical value N is about 22 dB, and the limit of the received electric field strength is required to be not lower than about −75 dBm. In a QPSK (Quadrature Phase Shift Keying) having the coding rate of 1/2 of the convolution code with a small transmission rate of 12 Mbps, on the other hand, the critical value N is about 6 dB, and the image can be transmitted with the critical value of the received electric field strength not lower than about −89 dBm. In the case where the transmission rate is low, the compression ratio is increased by MPEG processing, often resulting in an deteriorated image quality.
The environment of image transmission variously changes with the transmission distance as well as depending on whether the transmission is mobile or fixed. The user decides which is to be emphasized, the transmission rate or the transmission limit, in accordance with the transmission environment and determines a setting mode.
A digital FPU (Field Pick-up Unit), in order to meet this requirement, has four types of modulation settings including 64-QAM, 32-QAM, 16-QAM and QPSK, for example. Also, the convolution ratio associated with the intensity of error correction has five correction settings including 1/1 (no coding), 5/6, 3/4, 2/3 and 1/2, for example.
This mode is required to be set identically both at the transmitting and receiving ends.
These settings at the transmitting end can be automatically set at the receiving end if sent to the receiving end, thereby eliminating the need of the operation for sequentially setting the demodulation mode.
The ancillary data for automatic but not manual setting is transmitted using a carrier called TMCC (Transmission and Multiplexing Configuration Control) carrier.
This ancillary data is important information for transmission controlling, and therefore is transmitted by being subjected to BPSK (Binary Phase Shift Keying) modulation having a high transmission endurance.
At the receiving end, the TMCC carrier is demodulated, and by determining the setting mode of the main data, the setting mode for the main data receiver is set. Using this function, the mode at the receiving end can be automatically changed and set simply by only changing the setting at the transmitting end.
For the outline of 64-QAM, etc. and QPSK and error correction, refer, for example, to the Journal of the Institute of Image Information and Television Engineers, Vol. 52, No. 11, 1998, pp. 32 to 36.